The present invention relates to a drive circuit for and a method of operating a semiconductor laser, in particular a vertical cavity surface-emitting laser (VCSEL).
The drive circuit according to the invention is, in particular, suitable for driving various types of VCSEL, i.e. VCSELs having different characteristics.
VCSELs are semiconductor laser diodes that emit their light vertically with respect to the surface of the wafer. They have many advantages compared with other semiconductor lasers. Mention may be made, for example, of the very high modulation rates, the very low power consumption, a high efficiency for optical coupling to optical fibres, and also photolithographically defined geometries. In addition, VCSELs are inexpensive. Compared with DFB lasers, the cost factor is, for example, approximately 100. However, VCSELs have a limited output power and a marked temperature dependence. Application fields for VCSELs are, for example, 10 Gbit Ethernet networks, in-office systems and also transmission systems in the short-link field, such as Metro networks. VCSELs having a laser wavelength in the region of 850 nm have been known for a fairly long time. Recently, VCSELs having a laser wavelength of 1300 nm have also been marketed. In contrast to DFB lasers, which are normally operated in the continuous-wave mode and whose laser light is modulated for information transmission via an external downstream modulator, VCSELs are modulated directly.
A circuit for driving a VCSEL having an emission wavelength of 850 nm is known that is a differential amplifier. In this instance, one terminal of the VCSEL is connected to ground or chassis respectively and the other terminal of the VCSEL to an output of the differential amplifier. Such a connection arrangement is also termed “single-ended”. The VCSEL is operated with direct modulation by means of the differential amplifier. VCSELs having an emission wavelength of 850 nm have a relatively high threshold voltage of approximately 1.8 V, a threshold current of approximately 3 mA and a limit current of approximately 10 mA. Its differential resistance is in the order of magnitude of 60 Ω. To avoid distortions, the connecting line between the drive circuit and the VCSEL chip is normally matched to the differential resistance of the VCSEL.
The recently developed VCSELs having an emission wavelength of 1300 nm have a threshold voltage of 1.3 V but a differential resistance in the order of magnitude of 120 Ω. In contrast to connecting lines having an impedance of 120 Ω, the use of connecting lines having an impedance of 55-85 Ω does not present a problem from technical and economic points of view. The above-described drive circuit of the prior art has the disadvantage that, if in general only to a limited extent, it is usable for VCSELs having a high differential resistance.